1. Field of the Invention
The present invention relates to an iron-based powder useful for cleaning materials (particularly soil, underground water, etc.) contaminated with organic halides.
2. Description of the Related Art
Volatile organic chlorine compounds, such as trichloroethylene, have widely been used not only as degreasing detergents in semiconductor factories and metal working factories, but also as detergents in dry cleaning. However, those organic chlorine compounds have often been discharged and discarded in the past. Because the organic chlorine compounds are hard to decompose in the natural world, they are gradually accumulated in and contaminate the soil and the underground water, thus resulting in a social problem.
Known treatment methods for rendering contaminants in the soil and the underground water harmless include, for example, a pyrolysis method comprising the steps of excavating contaminated soil and removing the contaminants with combustion using a rotary kiln or the like, a soil vapor extraction method comprising the steps of sucking contaminants dissolved in the underground water with a vacuum pump and recovering the contaminants for removal, a pumping treatment method comprising the steps of pumping up the underground water and extracting contaminants for removal, and a microbial method utilizing the microbe ability of decomposing contaminants.
Those treatment methods, however, have disadvantages as follows. The pyrolysis method needs large-scaled equipment for excavating the soil and requires a high cost when the soil after the heat treatment is utilized for refilling. The soil vapor extraction method is able to recover only vaporized contaminants and raises the necessity of decomposing the contaminants after the recovery. The pumping treatment method is able to recover only contaminants dissolved in water and requires decomposition of the contaminants after the recovery. The microbial method cannot be applied to some cases depending on soil conditions. Further, because the microbial method relies on the decomposing reaction caused by microbes, there is a disadvantage that a longer treatment period than the other methods is needed in the case of high-concentration contamination, or the decomposing reaction proceeds just halfway in some cases.
A method of reducing and decomposing organic chlorine compounds using iron powder has been proposed as a method that requires neither large-scaled equipment nor the decomposing process after the recovery and can render the organic chlorine compounds harmless with stability. The proposed method utilizes electrons generated upon oxidization of the iron powder and decomposes the organic chlorine compounds while reducing them. However, the decomposition efficiency of the organic chlorine compounds with the iron powder is not so high because of the specific property, and hence a variety of methods are proposed for more satisfactory practical use.
For example, Japanese Examined Patent Application Publication No. 2-49158 proposes a method for decomposing hydrocarbon halides by adding iron powder to water, which is to be treated and contains hard-to-decompose hydrocarbon halides, and by shaking a mixture, the method comprising the steps of removing dissolved oxygen from the treated water in advance and adjusting the treated water to the pH range of 6.5 to 9.5. However, this method requires complicated operations, such as removal of the dissolved oxygen and the pH adjustment, and hence has a difficulty in application to field treatment in the contaminated site.
Japanese Patent Publication (by PCT Application) No. 6-506631 proposes a method for decomposing contaminated water containing organic chlorine compounds to render it harmless by passing the contaminated water through filings of iron and steel, the method comprising the steps of mixing the filings of iron and steel with activated coal, and passing the contaminated water through a mixture layer. However, this method necessitates the use of expensive activated coal and increases a treatment cost.
Recently, methods for increasing reactivity of iron powder itself without requiring pretreatment of contaminated soil and water or without employing activated coal have also been proposed. For example, Japanese Unexamined Patent Application Publication No. 11-235577 discloses a method comprising the steps of mixing soil with iron powder having a carbon content of not less than 0.1 weight % and a specific surface area of not less than 500 cm2/g, and decomposing organic chlorine compounds in the soil. The iron powder having a large specific surface area is provided as iron powder obtained by reducing iron ore (i.e., sponge-shaped ore-reduced iron powder). However, iron powder other than the sponge-shaped ore-reduced iron powder cannot be practically used because reactivity lowers when the iron powder has a smaller specific surface area.
Also, Japanese Unexamined Patent Application Publication No. 2001-9475 proposes the use of iron powder containing copper in consideration of that metallic copper noticeably promotes the reaction of decomposing organic halogen compounds, which is caused by metallic iron. However, copper has a high material cost. Further, when producing the iron powder containing copper, the proposal requires a complicated production process of mixing iron powder in a copper ion solution, such as an aqueous solution of copper sulfate, and recovering an obtained precipitate, whereby the production cost is increased.
In addition to the sponge-shaped iron powder mentioned above, atomized iron powder is also known. The atomized iron powder is produced by atomizing molten steel into powder. The as-atomized powder is called black powder because the powder surface is oxidized to become black. The black powder is perfectly reduced, as required, into a reduced powder. The reduced powder is called white powder in contrast with the black powder. The white powder is coated with a resin, which serves as a binder, over the powder surface, and is employed in the field of powder metallurgy. There is also proposed a method of utilizing the iron powder for powder metallurgy (i.e., the atomized iron powder) instead of the sponge-shaped iron powder for the purpose of cleaning soil.
For example, Japanese Unexamined Patent Application Publication No. 2002-20806 suggests a proposal of containing 0.01 to 4.0 mass % of Ni in the iron powder for powder metallurgy in consideration of that hydrogen produced on Ni surfaces promotes the decomposing reaction (including the reducing reaction of organic halogen contaminants and the oxidizing reaction of metals). However, this Publication states that reactivity of the cleaning reaction lowers when oxides are formed on powder surfaces.
In view of the state of the art described above, it is an object of the present invention to provide an iron-based cleaning powder capable of efficiently decomposing organic halides even when oxides are formed on powder surfaces.
Another object of the present invention is to provide an iron-based cleaning powder which is capable of decomposing organic halides at higher efficiency than the so-called white powder and can be produced in a more simple process than the white powder.
As a result of conducting intensive studies to achieve the above objects, the inventors discovered that, if the oxides formed on the powder surfaces are ones formed during atomization, even iron powder having those oxides is able to efficiently decompose organic halides on the contrary to the suggestion stated in the above-cited Japanese Unexamined Patent Application Publication No. 2002-20806, and then found that iron powder (black powder) obtained with the atomization process can be used for the purpose of cleaning materials (such as soil and underground water) contaminated with organic halides. Also, the inventors found that the effective use of the black powder eliminates the necessity of reducing the black powder to the white powder and enables cleaning iron powder to be provided with a simpler process. Further, the inventors found that, by preventing the oxidization in the atomization process as much as possible, resulting iron powder (comprising a martensite structure) is able to decompose organic halides at higher efficiency than the white powder (comprising a ferrite structure) obtained by reducing the resulting iron powder. Based on those findings, the present invention was accomplished. In order to obtain the effect decomposing, it is required that the iron powder comprises a martensite structure (including a tempered martensite structure).
To describe the present invention in detail, the inventors found that black powder (such as iron alloy powder and iron powder) can be effectively used for the purpose of cleaning materials (such as soil and underground water) contaminated with organic halides. The black powder is powder prepared with the atomization process and usually has an oxide coating formed on the powder surface at an H2-reduction mass loss on the order of not less than 0.15%. The black powder may be partially reduced so long as an oxide remains on the powder surface (i.e., so long as the H2-reduction mass loss is not less than 0.15%). Because the oxides on the surfaces of the black powder and the partially reduced powder thereof are ones formed during the atomization, those kinds of powder are able to effectively decompose organic halides in spite of the oxides remaining on the powder surfaces. The black powder comprises a martensite structure and the partially reduced power comprises a tempered martensite structure. Further, the black powder may be produced with atomization that is performed while preventing oxidization as much as possible (at the H2-reduction mass loss on the order of not less than 0.1%, but less than 0.15%). Non-oxidized black powder thus obtained has the amount of an oxide coating (reduction mass loss) comparable to that of the white powder obtained by reducing ordinary black powder, but it is capable of decomposing organic halides at higher efficiency than the white powder probably because of the fact that the non-oxidized black powder has a different metallic structure from that of the white powder (i.e., the non-oxidized black powder=martensite structure and the white powder=ferrite structure).
To summarize the above explanation, the oxide coating formed in the atomization process effects better in the decomposing reaction than that newly formed after the iron powder is reduced into a white powder, and the structure of a martensite or a tempered martensite effects better in the decomposing reaction than that of ferrite.
More specifically, an iron-based cleaning powder according to the present invention is made of iron alloy powder (Fexe2x80x94Mn alloy powder or Fexe2x80x94Ni alloy powder) or iron powder, which is prepared with an atomization process and has an oxide coating, or it is made of iron alloy powder (Fexe2x80x94Mn alloy powder or Fexe2x80x94Ni alloy powder) or iron powder, which is prepared with an atomization process without reduction. The Fexe2x80x94Mn alloy powder contains at least Mn in the range of 0.3 to 1.1% (mass %, this is similarly applied to % in the following), passes a 300 xcexcm-mesh sieve at a proportion of not less than 90%, has an H2-reduction mass loss of 0.1 to 0.8%, and has a martensite structure or a tempered martensite structure.
The Fexe2x80x94Ni alloy powder contains at least Ni in the range of 0.2 to 12%, passes a 300 xcexcm-mesh sieve at a proportion of not less than 90%, has an H2-reduction mass loss of 0.1 to 1.0%, and has a martensite structure or a tempered martensite structure.
The iron powder is used together with Ni-containing powder. Practically, as the iron-based cleaning powder, the iron powder is used in the form of mixed powder or partially alloyed powder made up of the iron powder and the Ni-containing powder. The iron powder passes a 300 xcexcm-mesh sieve at a proportion of not less than 90%, has an H2-reduction mass loss of 0.1 to 1.0%, and has a martensite structure or a tempered martensite structure. The Ni-containing powder has a Ni content of not less than 40% and passes a 45 xcexcm-mesh sieve at a proportion of not less than 90%. The Ni-containing powder usually has an H2-reduction mass loss of 0 to 1.0%. Preferably, a ratio of the iron powder to the Ni-containing powder (i.e., the iron powder/the Ni-containing powder) is in the range of 99.5/0.5 to 80/20 (mass ratio).
In the following description, the term xe2x80x9cblack powderxe2x80x9d means iron powder or iron alloy powder prepared with the atomization process, and is employed as including partially reduced powder on condition of excluding ordinary white power (powder of a ferrite structure). Also, the term xe2x80x9cblack powderxe2x80x9d is employed as including powder that has no oxide coatings on the powder surface, so long as the powder is essentially black powder from the standpoint of metallic structure (i.e., so long as the powder has a martensite structure or a tempered martensite structure).
According to the present invention, since a coating formed during the atomization is selected as an oxide coating formed on the surface of the iron-based powder, it is possible to decompose (reduce) organic halides even when an oxide is present on the powder surface (i.e., even when the black powder has the oxide coating). Therefore, the iron-based cleaning powder can be produced with a simpler process. In addition, when using non-oxidized black powder, the organic halides can be decomposed (reduced) at higher efficiency than the white powder. It effects advantageously as described above that the black powder (partially reduced black powder and non-oxidized black powder) comprises a martensite structure and a tempered martensite structure. As a result, the present invention enables the effective use of the black powder, which has been regarded as not being able to decompose the organic halides in the past.